Method for treating waste gas containing fluorochemical

ABSTRACT

A method for decomposing a fluorine-containing compound by contacting an exhaust gas  1  containing a fluorine-containing compound, such as C 2 F 6 , with aluminum oxide  3 , such as γ-alumina, which has been heated to a high temperature of, for example, 800° C. to 900° C. There may be such a necessary or larger number of moles of H 2  that the fluorine atoms in the fluorine-containing compound become HF. The method has a high decomposition rate of a fluorine-containing compound, a long life, and involves minimal occurrence of CO.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for treating an exhaust gas containing a fluorine-containing compound. More particularly, the invention relates to a treatment method and a treatment apparatus for efficiently decomposing C₂F₆, C₃F₈, CF₄, CHF₃, SF₆ or NF₃ which is discharged in the step of dry cleaning the internal surface, etc. of a chamber or the like of semiconductor manufacturing equipment with the use of C₂F₆, C₃F₈, CF₄, CHF₃, SF₆ or NF₃ in the semiconductor industry, or during etching of various films on a semiconductor device.

PRIOR ART

In the semiconductor industry, many kinds of hazardous gases are used in the semiconductor manufacturing process, arousing concern for environmental contamination by them. C₂F₆, contained in an exhaust gas from a chemical vapor deposition (CVD) step and an etching step, acts as a global warming gas, and the establishment of a system for its removal is considered to be an urgent task.

In dry etching a silicon wafer or the like with the use of a manufacturing apparatus for a semiconductor device, such as an etching apparatus, for example, fluorinated hydrocarbons, such as CF₄, CHF₃, and C₂F₆, are used. In an exhaust gas from the etching apparatus, fluorinated hydrocarbons, such as CF₄, CHF₃, and C₂F₆, are contained.

An exhaust gas containing a fluorine-containing compound may be discharged during cleaning of a manufacturing apparatus for a semiconductor device. To form a thin film on a semiconductor substrate, a chemical vapor deposition apparatus is used as a semiconductor device manufacturing apparatus. When thin films adhering to a chamber and an internal surface of piping in the chemical vapor deposition apparatus are cleaned with a ClF₃ gas, an exhaust gas including various fluorine-containing gases is discharged from the chemical vapor deposition apparatus.

Various decomposition techniques and recovery techniques have been proposed as methods for treating fluorine-containing compounds such as C₂F₆. As the decomposition technology, in particular, the use of compounds, such as Pt catalysts, zeolite-based catalysts, activated carbon, active alumina, alkali metals, alkaline earth metals, and metal oxides, is named as a method of catalytic thermal decomposition. However, none of methods using them have been found effective.

Of the conventional technologies, the method of catalytic thermal decomposition comprises decomposing C₂F₆ by passing it through a heated catalyst. However, all of the methods of this type have involved drawbacks, such as a low decomposition rate, a short life of the treating agent, and occurrence of carbon monoxide (CO) as a by-product which is difficult to treat.

Under these circumstances, the present invention aims to solve the above-described problems with the earlier technologies, and provide a method and an apparatus for treating an exhaust gas containing a fluorine-containing compound, the method and apparatus having a high decomposition rate, being effective for long periods, and involving minimal occurrence of carbon monoxide (CO).

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a method for decomposing a fluorine-containing compound in an exhaust gas by contacting the exhaust gas containing the fluorine-containing compound with aluminum oxide at a high temperature enough to decompose the fluorine-containing compound.

In the present invention, the above contacting step is preferably performed in the presence of a hydrogen gas (H₂) in a molar amount in excess of the amount required to convert the fluorine atoms in the fluorine-containing compound to HF. More preferably, the contacting step is performed in the presence of 1.5 or more times as many as the molar amount of a hydrogen gas (H₂) required to convert the fluorine atoms in the fluorine-containing compound to HF.

Preferably, the contacting step is performed in the presence of an oxygen gas whose amount is not less than the number of moles of the hydrogen gas.

Preferably, the fluorine-containing compound contains a carbon atom, a sulfur atom, or a nitrogen atom, and the contacting step is performed in the presence of an oxygen gas (O₂) in a molar amount in excess of the amount required to convert the carbon atom to CO₂, the sulfur atom to SO₂, or the nitrogen atom to NO₂.

The aluminum oxide preferably includes γ-alumina. Preferably, the aluminum oxide is particulate.

The high temperature is preferably in the temperature range of about 800° C. to about 900° C.

The fluorine-containing compound preferably includes C₂F₆, C₃F₈, CF₄, CHF₃SF₆ or NF₃. The exhaust gas preferably has been discharged from a semiconductor device manufacturing apparatus.

It is preferred to remove a catalytic poison to the aluminum oxide from the exhaust gas, and then perform the contacting step. It is further preferred to remove SiF₄ from the exhaust gas, and then perform the contacting step.

Preferably, the step of removing an acidic gas is present after the contacting step. Further preferably, the acidic gas is removed using water.

According to another aspect of the present invention, there is provided an apparatus for treating an exhaust gas containing a fluorine-containing compound, which includes a vessel for accommodating aluminum oxide, and a passage for passing the exhaust gas containing the fluorine-containing compound into the vessel.

In the present invention, it is preferred to further include a heater capable of heating the aluminum oxide to a high temperature enough to decompose the fluorine-containing compound.

It is preferred to further include a feeder for feeding a hydrogen gas (H₂) in a molar amount in excess of the amount required to convert the fluorine atoms in the fluorine-containing compound to HF.

Preferably, a feeder for feeding an oxidizing agent is further included.

Preferably, the fluorine-containing compound contains a carbon atom, a sulfur atom, or a nitrogen atom, and there is further included a feeder for feeding O₂ in a molar amount in excess of the amount required to convert the carbon atom to CO₂, the sulfur atom to SO₂, or the nitrogen atom to NO₂.

The aluminum oxide preferably includes γ-alumina. More preferably, the aluminum oxide is particulate.

The passage is preferably connected to an outlet of a semiconductor device manufacturing apparatus.

The vessel is preferably a packed column.

Preferably, a pretreatment apparatus for removing a catalytic poison to the aluminum oxide from the exhaust gas is further present upstream from the vessel. Also preferably, a pretreatment apparatus for removing SiF₄ from the exhaust gas is further present upstream from the vessel.

Preferably, a posttreatment apparatus for removing an acidic gas is further placed downstream from the vessel. The posttreatment apparatus preferably removes the acidic gas by use of water.

According to still another aspect of the present invention, there is provided a method for producing a semiconductor device, including the steps of:

etching a precursor of the semiconductor device with an etching gas containing a fluorine-containing compound or a plasma thereof in a chamber;

discharging an exhaust gas containing a fluorine-containing compound from the chamber; and

contacting the exhaust gas with aluminum oxide at a high temperature enough to decompose the fluorine-containing compound, thereby decomposing the fluorine-containing compound in the exhaust gas.

Preferably, the contacting step is performed in the presence of a hydrogen gas (H₂) in a molar amount in excess of the amount required to convert the fluorine atoms in the fluorine-containing compound to HF.

According to a further aspect of the present invention, there is provided a method for producing a semiconductor device, including the steps of:

forming a thin film in a chamber of a chemical vapor deposition apparatus by chemical vapor deposition of a thin film-forming gas on a precursor of the semiconductor device;

withdrawing the resulting semiconductor device from the chamber;

cleaning the chamber of the chemical vapor deposition apparatus with a cleaning gas containing a fluorine-containing compound; and

contacting an exhaust gas containing a fluorine-containing compound from the cleaning step with aluminum oxide at a high temperature enough to decompose the fluorine-containing compound, thereby decomposing the fluorine-containing compound in the exhaust gas.

Preferably, the contacting step is performed in the presence of a hydrogen gas (H₂) in a molar amount in excess of the amount required to convert the fluorine atoms in the fluorine-containing compound to HF.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of an apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory view of an apparatus according to another embodiment of the present invention.

FIG. 3 is an explanatory view of an apparatus according to still another embodiment of the present invention.

FIG. 4 is an explanatory sectional view of an apparatus according to a further embodiment of the present invention.

FIG. 5 is an explanatory view of an embodiment of an etching apparatus.

FIG. 6 is an explanatory view of another embodiment of an etching apparatus.

FIG. 7 is an explanatory view of an apparatus according to a still further embodiment of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

According to an aspect of the present invention, a fluorine-containing compound is decomposed at a high temperature with the use of aluminum oxide, e.g., γ-alumina, as a catalyst. For example, a perfluorocarbon, such as C₂F₆, C₃F₈ or CF₄, can be decomposed. Also, an organic fluorine compound such as CHF₃, or an inorganic fluorine compound such as SF₆ or NF₃ can be decomposed. That is, the fluorine-carbon bond, fluorine-sulfur bond, or fluorine-nitrogen bond in the fluorine-containing compound can be cleaved.

The fluorine-containing compound includes a chlorofluorocarbon such as C₂Cl₃F₃, C₂Cl₂F₄, or C₂ClF₅. Such a chlorofluorocarbon can also be decomposed. However, the fluorine-containing compound is preferably free from a chlorine atom, and is more preferably free from any of a chlorine atom, a bromine atom, and an iodine atom.

According to an embodiment of the present invention, the decomposition efficiency for the fluorine-containing compound is increased markedly by adding an H₂ gas, particularly an H₂ gas and an O₂ gas, into the exhaust gas. By using an H₂ gas, the fluorine-containing compound can be decomposed into HF.

An embodiment in which an H₂ gas and an O₂ gas are both added will be described mainly.

When a fluorine-containing compound, such as C₂F₆, C₃F₈ or CF₄, is contacted with γ-alumina, which has been heated to a high temperature, in the presence of sufficient amounts of a hydrogen gas and an oxygen gas, this compound is decomposed into CO₂ and HF in accordance with the following reaction scheme:

 C₂F₆+3H₂+3O₂→2CO₂+6HF+O₂

C₃F₈+4H₂+4O₂→3CO₂+8HF+O₂

CF₄+2H₂+2O₂→CO₂+4HF+O₂

When SF₆ or NF₃ is subjected to a similar reaction in an oxidizing atmosphere having an excess of oxygen, the following reaction takes place to decompose the compound:

SF₆+3H₂+2O₂→SO₂+6HF+O₂

2NF₃+3H₂+3O₂→2NO₂+6HF+O₂

When CHF₃ is subjected to a similar reaction in an oxidizing atmosphere having an excess of oxygen, the following reaction takes place to decompose the compound:

CHF₃+H₂+2O₂→CO₂+3HF+O₂

In view of the above findings, in an embodiment of the present invention, a fluorine-containing compound is brought into contact with aluminum oxide in the presence of a hydrogen gas (H₂) in a molar amount in excess of the amount required to convert the fluorine atoms in the fluorine-containing compound to HF. If the fluorine-containing compound contains an H atom, for example, like CHF₃, the hydrogen gas may be added such that the total number of the hydrogen atom in the fluorine-containing compound and the hydrogen atoms in the hydrogen gas added (H₂) is not less than the number of moles of the fluorine atoms in the fluorine-containing compound. More preferably, the contacting step is performed in the presence of 1.5 or more times as many as the molar amount of a hydrogen gas (H₂) required to convert the fluorine atoms in the fluorine-containing compound to HF.

Even when H₂ is present in an amount sufficient to react with the fluorine atoms in the fluorine-containing compound, if the amount of O₂ added is not more than the number of moles of the carbon atoms, C₂F₆ is decomposed into CO₂ and HF, and a large amount of CO is also produced, for example, according to the following scheme:

2C₂F₆+6H₂+3O₂→2CO₂+2CO+12HF

In view of the above facts, in an embodiment of the present invention, it is preferred that decomposition is performed in the presence of O₂ in a molar amount in excess of the amount required to convert the carbon atom in the fluorine-containing compound to CO₂, the sulfur atom in the fluorine-containing compound to SO₂, or the nitrogen atom in the fluorine-containing compound to NO₂. It is more preferred that decomposition is performed in the presence of O₂ in an amount of 1.5 or more times as many as this number of moles.

As the aluminum oxide, γ-alumina, δ-alumina or θ-alumina can be used, and γ-alumina is preferred. The aluminum oxide may contain a trace amount of silicon, boron, or a transition metal such as iron, chromium, nickel, palladium, or platinum. The surface of the aluminum oxide may be treated with a coating.

The shape of the aluminum oxide is not restricted, and may be particulate, powdery, or honeycomb-like. If it is particulate or powdery, a spherical particulate or powdery shape is preferred when handled.

The particle size of the aluminum oxide is in a range in which air flow resistance does not increase when the exhaust gas is passed through the aluminum oxide. To impart a large contact area, fine particles of the aluminum oxide are satisfactory. For example, the desirable particle size is 0.8 to 2.6 mm.

The specific surface area of the alumina is preferably 100 to 400 m²/g, and more preferably 120 to 300 m²/g.

The temperature of the aluminum oxide may be a high temperature at which its catalytic activity is obtained. The temperature depends on the fluorine-containing compound, and in the case of C₂F₆, C₃F₈, CF₄, CHF₃, SF₆ or NF₃, for example, the range of 800 to 900° C. is preferred. For a chlorofluoro compound with the fluorine atoms substituted by chlorine atoms, the reaction temperature may be lower, as the number of the chlorine atoms increases.

The apparatus according to the embodiments of the present invention is shown in FIGS. 1 to 3. In FIG. 1, H₂ and O₂ are introduced into an exhaust gas 1 separately. In FIG. 2, H₂ and O₂ are introduced into an exhaust gas 1 simultaneously. In FIG. 3, an exhaust gas 1, H₂ and O₂ are introduced into a packed column 2 separately. In these drawings, the numeral 3 denotes γ-alumina, and the numeral 4 denotes a heater.

As described above, H₂ may be added to an exhaust gas, and then O₂ may be added to the exhaust gas. Alternatively, both H₂ and O₂ may be added to an exhaust gas. Furthermore, an exhaust gas, H₂ and O₂ may be introduced into a packed column 2 at a time, and the exhaust gas, H₂ and O₂ may be mixed inside the packed column 2.

A feeder for feeding H₂ includes, for example, a high pressure cylinder for storing a hydrogen gas, and a regulator for controlling the pressure and flow rate of the hydrogen gas fed. A feeder for feeding O₂ includes, for example, a high pressure cylinder for storing an oxygen gas, and a regulator for controlling the pressure and flow rate of the hydrogen gas fed.

In FIGS. 1, 2 and 3, a mixed gas containing an H₂ gas and an O₂ gas and an exhaust gas are introduced into the packed column 2 as downward streams. However, they may be introduced as upward streams, rather than downward streams.

The exhaust gas after having passed through the packed column 2 contains HF. Thus, the exhaust gas is released to the atmosphere after removal of HF by such a measure as its passage through a water scrubber tower already installed. Also, carbon monoxide (CO) is decomposed and removed, if desired, by means of an oxidation catalyst or the like.

FIG. 4 shows an embodiment of the apparatus of the present invention. In the apparatus, a passage 16 for introducing an exhaust gas containing a fluorine-containing compound into a vessel 10 is connected to a top 10 a of the vessel 10, and a passage 18 for discharging a treated gas from the vessel 10 is connected to a bottom 10 b of the vessel 10. The passage 16 is connected, for example, to an outlet of a semiconductor device manufacturing apparatus.

The vessel 10 includes a support floor 11 provided in a lower portion thereof, aluminum oxide 12 supported on the support floor 11, a first gap 13 above the aluminum oxide 12, and a second gap 14 below the aluminum oxide 12. The support floor 11 has many small holes of such a size as to allow the exhaust gas to pass therethrough, but does not allow the aluminum oxide 12 to pass therethrough. The support floor is, for example, a plate-like member composed of stainless steel. The gap 13 and the second gap 14 are of a sufficient size to allow the exhaust gas or mixed gas fed through the passage 16 to flow into and from the aluminum oxide 12 at a constantly desired speed.

Unlike the embodiment of FIG. 4, the exhaust gas may be introduced through the passage 18, and the treated gas may be discharged through the passage 16.

Outside the vessel 10, a heater 20 capable of heating the aluminum oxide 12 is provided. There is no restriction on the heater 20. The heater may be an electric heater, or the aluminum oxide may be heated with a heating medium, or the outside of the packed column 2 may be heated by a burner.

The manufacturing apparatus for the semiconductor device is, for example, an etching apparatus or a chemical vapor deposition (CVD) apparatus. The exhaust gas generated by the manufacturing apparatus for the semiconductor device includes an exhaust gas produced in a semiconductor device manufacturing process, and an exhaust gas generated during cleaning of the manufacturing apparatus after production of the semiconductor device.

The semiconductor device is not limited, and includes diodes, transistors, thyristors, memories such as ROM and RAM, and CPU.

The precursor of the semiconductor device includes, for example, a substrate comprising a semiconductor, and a precursor having a thin film laminated on the substrate. The semiconductor of the substrate includes Group IV elements such as silicon, and compound semiconductors such as III-V compound semiconductors and II-VI compound semiconductors.

During etching of the precursor of the semiconductor device with an etching gas or its plasma, an exhaust gas is discharged. The etching step in the method of manufacturing a semiconductor device is described, for example, in Japanese Patent Publication No. 14151/1981, and Japanese Patent Publication No. 45310/1982. The entire disclosure of Japanese Patent Publication No. 14151/1981 and Japanese Patent Publication No. 45310/1982 is incorporated herein by reference. Furthermore, semiconductor manufacturing methods and semiconductor devices are described in Jacqueline I. Kroschwitz, Mary Howe-Grant, “Kirk-Othmer Encyclopedia of Chemical Technology”, Ed. 4, Vol. 21, 720-816, John Wiley & Sons, Inc., 1997. The entire description of this book is cited herein. Semiconductor devices are also described in M. S. Tyagi, “Introduction to Semiconductor Materials and Devices,” John Wiley & Sons, Inc., 1991, pp. 299-562. These descriptions are incorporated in the specification by reference. Manufacturing methods for semiconductor devices are described in “Introduction to Semiconductor Materials and Devices,” pp. 563-612. These descriptions are also incorporated in the specification by reference.

The etching includes sputter etching for etching by physical impact of an etching gas, reactive ion etching for etching by the synergistic effect of physical impact of an etching gas and a chemical reaction with the etching gas, and plasma etching for etching by a chemical reaction with an etching gas without substantially involving physical impact of an etching gas.

In sputter etching, a fluorinated hydrocarbon such as CF₄, CHF₃ or C₂F₆ is used preferably as an etching gas. In reactive ion etching and plasma etching, BCl₃, BF₃ or Cl₂ is used preferably as an etching gas.

FIG. 5 is an explanatory view of an embodiment of an etching apparatus. The etching apparatus has a vacuum chamber 50, and an electrode 52 placed inside the vacuum chamber 50. The electrode 52 is connected to a high frequency voltage 58. On the electrode 52, a precursor 62 of a semiconductor device, for example, a semiconductor wafer is placed. A high frequency voltage is applied to the electrode 52, with an etching gas 64 flowing into the vacuum chamber 50, whereby sputter etching or reactive ion etching of the precursor 62 of the semiconductor device is performed. In this case, the type of the etching gas 64 determines whether sputter etching or reactive ion etching is performed. An exhaust gas 66 is discharged from the vacuum chamber, and introduced into an exhaust gas treatment apparatus.

In FIG. 5, a single electrode is used. However, a pair of electrodes 54 and 56 may be used, as shown in FIG. 6. In FIG. 6, a high frequency voltage 58 is applied to an electrode 54 and an electrode 56, with an etching gas 64 flowing into a vacuum chamber 50, whereby sputter etching or reactive ion etching is performed for a precursor 62 of a semiconductor device placed on the electrode 54. An exhaust gas 66 is discharged from the vacuum chamber, and introduced into an exhaust gas treatment apparatus.

FIG. 7 is an explanatory view of another embodiment of the present invention. An exhaust gas containing a fluorine-containing compound, which has been discharged from a semiconductor device manufacturing apparatus or the like, is introduced via a line 42 into a pretreatment apparatus 32 for removing a fluorine-containing inorganic compound, etc. Then, the exhaust gas is introduced via a line 44 into a packed column 10 containing aluminum oxide. Then, the treated gas is introduced via a line 46 into a posttreatment apparatus 34 for removing an acidic gas such as HF, and discharged through a line 48. As the packed column 10, that shown, for example, in FIG. 4 can be used.

The pretreatment apparatus is, for example, a dry apparatus charged with a solid chemical such as zeolite, or a wet apparatus using a liquid chemical.

The exhaust gas discharged from the semiconductor device manufacturing apparatus may contain tetrafluorosilicon (SiF₄) and an oxygen gas (O₂) along with tetrafluorocarbon (CF₄). Tetrafluorosilicon (SiF₄) and an oxygen gas (O₂) react with each other to form solid silicon oxide (SiO₂). That is, when the exhaust gas containing tetrafluorosilicon (SiF₄) and an oxygen gas (O₂) along with tetrafluorocarbon (CF₄) is contacted with aluminum oxide, solid silicon oxide (SiO₂) may be formed on the surface of the aluminum oxide, lowering the ability of the aluminum oxide to decompose a fluorine-containing compound.

Thus, it is preferred to decompose and remove a catalytic poison in the exhaust gas, such as SiF₄, in the pretreatment apparatus before the exhaust gas is brought into contact with aluminum oxide. For example, the exhaust gas is passed through a packed column packed with a solid chemical such as zeolite, whereby SiF₄ can be decomposed and removed.

Alternatively, a fluorine-containing compound such as HF can be removed by spraying the exhaust gas with water, preferably, an alkaline liquid chemical, or by introducing the exhaust gas into water, preferably, an alkaline liquid chemical and bubbling it.

Apparatuses usable as the pretreatment apparatus 32 are described, for example, in Japanese Patent Application No. 168572/1998 entitled “Method for Treating Exhaust Gas Containing Inorganic Halogenated Gas.” The entire disclosure of this application is incorporated herein by reference.

The aluminum oxide in the packed column 10 decomposes a fluorine-containing compound, especially a fluorine-containing organic compound, into hydrogen fluoride (HF). Thus, the exhaust gas discharged from the packed column 10 contains a large amount of hydrogen fluoride. Hence, it is preferred to remove HF by a posttreatment apparatus.

The posttreatment apparatus for removing an acidic gas such as HF is, for example, a dry apparatus such as a packed column packed with an anion exchange resin, or a wet apparatus using water. The wet apparatus is, for example, a sprayer for spraying the exhaust gas with water, preferably, an alkaline liquid chemical, or a device for introducing the exhaust gas into water, preferably, an alkaline liquid chemical and bubbling it.

The spraying device and the bubbling device as the pretreatment apparatus and the posttreatment apparatus are described, for example, in WO99/24151 and WO99/20374. The entire disclosure of these international publications is incorporated herein by reference.

EXAMPLES

The present invention will now be described concretely by way of Examples, but the invention is not restricted thereby.

Reference Examples 1 to 4

A quartz column with a diameter of 25 mm was used, and packed with a treating agent to a layer height of 100 mm.

The packed column was mounted in a ceramic electric tubular furnace, and the treating agent layer was heated to 700° C. A gas of C₂F₆ alone diluted with an N₂ gas was passed through the treating agent layer. The flow rate of the gas was 408 sccm, and the SV was set at 500 hr⁻¹. The inlet gas concentration of C₂F₆ was adjusted to 1%.

To investigate the treating performance, the outlet gas was analyzed, where necessary. When the removal rate of C₂F₆ lowered to 90% or less, passage of the gas was stopped, and the amount of C₂F₆ treated was determined based on the amount of the gas passed by then. The analysis of C₂F₆ was made using a gas chromatograph equipped with a TCD detector.

The treating agents used were all commercially available products of Mizusawa Kagaku. The treating agents were γ-alumina (Neobead GB-26) in Reference Example 1, HY zeolite (Mizuka Sieves Y-520) in Reference Example 2, H mordenite (H Mordenite) in Reference Example 3, and H-ZSM-5 zeolite (Mizuka Sieves EX-122-H) in Reference Example 4. In any of the Reference Examples, the shape of the treating agent was particulate, and the particle sizes were 2.6 mm, 1 mm, 1 mm, and 1.5 mm, respectively. The results are shown in Table 1.

TABLE 1 Amount of C₂F₆ treated Treating Agent (liter/liter) Reference Example 1 γ-alumina 8.0 Reference Example 2 HY zeolite 0.1 Reference Example 3 H mordenite 2.5 Reference Example 4 H-ZSM-5 zeolite 1.3

The γ-alumina of Reference Example 1 was higher in performance than the H zeolites of Reference Examples 2 to 4.

Example 1 and Comparative Examples 1 to 3

The same testing equipment as in the preceding Reference Examples was used, and the column was packed with only γ-alumina as the treating agent. The amount packed and the particle diameter were the same, and O₂ and H₂ as gases added were mixed, in arbitrary proportions, with C₂F₆ diluted with an N₂ gas. In Example 1, the amount of H₂ added was set such that the H atomic weight was at an equal atomic ratio to the fluorine atomic weight of C₂F₆, and the amount of O₂ added was not less than equimolar relative to H₂ introduced.

In Comparative Examples 1 and 2, the gases added were O₂ alone and H₂ alone, respectively. In Comparative Example 3, O₂ and H₂ were both added, but the amount of H₂ was in excess of O₂. The treating temperature was 800° C. in all of the Examples. The analysis of C₂F₆, O₂, H₂, CO and CO₂ in the inlet gas and the outlet gas was quantitatively made by using a gas chromatograph equipped with a TCD detector. To investigate the treating performance, the outlet gas was analyzed, where necessary. When the removal rate of C₂F₆ lowered to 95% or less, passage of the gas was stopped, and the amount of C₂F₆ treated was determined based on the amount of the gas passed by then.

The results are shown in Table 2 below. The concentrations of the respective components in the outlet gas are values at a C₂F₆ removal rate of 95%.

In Comparative Example 1, H₂ was not added, and the reaction in this case proceeds as follows:

2Al₂O₃ (γ-alumina)+2C₂F₆+2)₂→4CO₂+4AlF₃+O₂

As noted above, the catalyst, γ-alumina, changes into AlF₃, so that the catalytic activity declines and the life of the catalyst shortens. This reaction also occurs when the amount of H₂ added is smaller than the fluorine atomic weight of C₂F₆. In this case, the catalyst, γ-alumina, changes, and the catalytic activity lowers. On the other hand, when H₂ is added in an equal amount or more, the fluorine atoms in C₂F₆ become HF. Thus, AlF₃ does not form, the catalyst, γ-alumina, does not react, and the catalytic activity does not decline.

In Example 1, H₂ and O₂ were added in the aforementioned mixing ratio. As a result, the amount of carbon monoxide (CO) formed was decreased, and the amount of C₂F₆ dramatically increased to 117 liters/liter. In Comparative Example 1 with the addition of O₂ alone, generation of carbon monoxide (CO) was decreased, but the life of the catalyst was short. When H₂ was added alone, or H₂ was introduced in an amount exceeding that of O₂, carbon monoxide (CO) occurred in a large amount, and the amount of treatment was small.

Examples 2, 3 and Comparative Example 4

The same testing equipment as in the preceding Examples was used, and γ-alumina was used. The particle diameter was 0.8 mm, and the same amount packed was employed. H₂ and O₂ as gases added were added to C₂F₆ diluted with an N₂ gas. That is, the amount of H₂ was set such that the hydrogen atoms had an equal atomic ratio to the fluorine atoms of C₂F₆, and the amount of O₂ gas was not less than equimolar relative to the hydrogen gas introduced. In Examples 2 and 3, the temperature of the treating agent layer was 800° C. and 900° C., respectively. In Comparative Example 4, the temperature was lowered to 700° C.

The analysis of the respective components in the inlet gas and the outlet gas was made by gas chromatography. C₂F₆ in the outlet gas was analyzed where necessary, and the amount of C₂F₆ treated was determined based on the amount of the gas passed, when the removal rate of C₂F₆ lowered to 95% or less.

The results are shown in Table 2 below. The concentrations of the respective components in the outlet gas are values at a C₂F₆ removal rate of 95%.

In Examples 2 and 3, in which C₂F₆ was treated at the temperature of 800° C. and 900° C., occurrence of carbon monoxide (CO) was minimal, and the amount of treatment was large. In Comparative Example 4, in which the treating temperature was lowered to 700° C., the ability to decompose C₂F₆ considerably lowered.

TABLE 2 Treating Concentrations Concentrations Amount agent layer Total gas in inlet gas in outlet gas of C₂F₆ Treating temp. inflow C₂F₆ O₂ H₂ C₂F₆ CO CO₂ treated agent (° C.) (sccm) (%) (%) (%) (ppm) (ppm) (ppm) (l/l) Ex. 1 γ-alumina 800 413 1.13 5.2 3.5 630  140 15800 117 Comp. Ex. 1 γ-alumina 800 418 1.32 3.5 0 690  190 17700 6.8 Comp. Ex. 2 γ-alumina 800 400 1.45 0 7.6 770 10000   5000 7.1 Comp. Ex. 3 γ-alumina 800 418 1.41 3.4 7.4 800 9300  9200 11 Ex. 2 γ-alumina 800 408 1.16 5.7 3.6 580  90 23200 120 Ex. 3 γ-alumina 900 408 1.18 5.6 3.5 590  70 23600 100 Comp. Ex. 4 γ-alumina 700 408 1.16 5.8 3.5 580 1500 16300 5.8

Examples 4, 5 and Comparative Example 5

The same testing equipment as in the preceding Example 1 was used, and γ-alumina was used. The particle diameter was 0.8 mm, and the same amount packed was employed. H₂ and O₂ as gases added were added to C₃F₈ diluted with an N₂ gas. The amount of H₂ was set such that the hydrogen atoms had an equal atomic ratio to the fluorine atoms of C₃F₈, and the amount of O₂ was not less than equimolar relative to the amount of H₂ introduced. In Examples 4 and 5, the temperature of the treating agent layer was 800° C. and 900° C., respectively. In Comparative Example 5, the temperature was 700° C. The analysis of the respective components in the inlet gas and the outlet gas was made by gas chromatography. C₃F₈ in the outlet gas was analyzed where necessary, and the amount of C₃F₈ treated was determined based on the amount of the gas passed, when the removal rate of C₃F₈ lowered to 95% or less.

The results are shown in Table 3 below. The concentrations of the respective components in the outlet gas are values at a C₃F₈ removal rate of 95%.

The present method of treatment was found to exhibit high performance for C₃F₈.

Comparative Examples 6 to 10

The same testing equipment and treating agents as used in Tables 1 and 2 were used. The amount packed and the particle diameter were the same, and O₂ and H₂ as gases added were mixed, in arbitrary proportions, with C₃F₈ diluted with an N₂ gas.

In Comparative Examples 6 to 8, γ-alumina was used. In Comparative Examples 9 and 10, H mordenite and H-ZSM-5 zeolite, which showed the next best performance after γ-alumina, were used. In Comparative Examples 6 and 7, the gases added were O₂ alone and H₂ alone, respectively. In Comparative Example 8, the amount of H₂ added was in excess of that of O₂. In Comparative Examples 9 and 10, the amount of H₂ added was set such that the H atomic weight was at an equal atomic ratio to the fluorine atomic weight of C₃F₈, and the amount of O₂ added was not less than equimolar relative to the amount of H₂ introduced.

The treating temperature was 800° C. The analysis of the respective components in the inlet gas and the outlet gas was made by gas chromatography. C₃F₈ in the outlet gas was analyzed where necessary, and the amount of C₃F₈ treated was determined based on the amount of the gas passed, when the removal rate of C₃F₈ lowered to 95% or less.

The results are shown in Table 3 below. The concentrations of the respective components in the outlet gas are values at a C₃F₈ removal rate of 95%.

Upon decomposition of C₃F₈ with γ-alumina, the amounts treated were not large, when O₂ was added alone, H₂ was added alone, and H₂ was added in an amount exceeding the amount of O₂. Even when H₂ and O₂ were mixed in the optimum ratio, the treating performance of H-type zeolites, such as H mordenite and H-ZSM-5 zeolite, other than γ-alumina was low.

In Comparative Example 6, γ-alumina changes into AlF₃ according to the following reaction scheme, resulting in its deterioration:

4Al₂O₃+3C₃F₈+4O₂→9CO₂+8AlF₃+O₂

When γ-alumina deteriorates, its catalytic activity declines and its life shortens.

On the other hand, when H₂ is added such that its atomic weight is not less than the fluorine atomic weight of C₃F₈, the fluorine atoms in C₃F₈ become HF. Thus, no AlF₃ forms, and there is no decline in the catalytic activity of γ-alumina.

TABLE 3 Treating Concentrations Concentrations Amount agent layer Total gas in inlet gas in outlet gas of C₂F₆ Treating temp. inflow C₃F₈ O₂ H₂ C₃F₈ CO CO₂ treated agent (° C.) (sccm) (%) (%) (%) (ppm) (ppm) (ppm) (l/l) Ex. 4 γ-alumina 800 408 1.14 5.6 4.6 570   90 34100 110 Ex. 5 γ-alumina 900 408 1.15 5.6 4.6 580   65 34300 95 Comp. Ex. 5 γ-alumina 700 408 1.14 5.7 4.6 270  2600 23700 5.0 Comp. Ex. 6 γ-alumina 800 408 1.14 5.6 0 570  180 26100 6.4 Comp. Ex. 7 γ-alumina 800 408 1.15 0 7.5 580 11500 11000 7.0 Comp. Ex. 8 γ-alumina 800 408 1.14 3.8 7.6 570 11000 13500 11 Comp. Ex. 9 H mordenite 800 408 1.15 5.6 4.6 580 20000 15000 6.5 Comp. Ex. 10 H-ZSM-5 800 408 1.14 5.6 4.6 570  1500 13500 4.3 zeolite

Examples 6, 7 and Comparative Example 11

The same testing equipment as in Example 1 was used. The particle diameter of γ-alumina was 0.8 mm, and the same amount packed was employed. H₂ and O₂ as gases added were added to CF₄ diluted with an N₂ gas. The amount of H₂ added was such that the H atoms was at an equal atomic ratio to the F atoms of CF₄, and the amount of O₂ gas added was not less than equimolar relative to the amount of H₂ gas. In Examples 6 and 7, the temperature of the treating agent layer was 800° C. and 900° C., respectively. In Comparative Example 11, the temperature was 700° C. The analysis of the respective components in the inlet gas and the outlet gas was made by gas chromatography. CF₄ in the outlet gas was analyzed where necessary, and the amount of CF₄ treated was determined based on the amount of the gas passed, when the removal rate of CF₄ lowered to 95% or less.

The results are shown in Table 4 below. The concentrations of the respective components in the outlet gas are values at a CF₄ removal rate of 95%.

The present method of treatment was found to exhibit high performance for CF₄.

Comparative Examples 12 to 16

The same testing equipment and treating agents as used in Tables 1 and 2 were used. The amount packed and the particle diameter were the same, and O₂ gas and H₂ gas in arbitrary proportions were mixed with CF₄ diluted with an N₂ gas.

In Comparative Examples 12 to 14, γ-alumina was used. In Comparative Examples 15 and 16, H mordenite and H-ZSM-5 zeolite, which showed the next best performance after γ-alumina, were used. In Comparative Examples 12 and 13, the gases added were O₂ alone and H₂ alone, respectively. In Comparative Example 14, the amount of H₂ added was in excess of that of O₂. In Comparative Examples 15 and 16, the amount of H₂ added was set such that the H atomic weight was at an equal atomic ratio to the fluorine atomic weight of CF₄, and the amount of O₂ gas added was not less than equimolar relative to the amount of H₂ gas introduced.

The treating temperature was 800° C. The analysis of the respective components in the inlet gas and the outlet gas was made by gas chromatography. CF₄ in the outlet gas was analyzed where necessary, and the amount of CF₄ treated was determined based on the amount of the gas passed, when the removal rate of CF₄ lowered to 95% or less.

The results are shown in Table 4 below. The concentrations of the respective components in the outlet gas are values at a CF₄ removal rate of 95%.

Upon decomposition of CF₄ with γ-alumina, the amounts treated were not large, when the gas added was O₂ alone (Comparative Example 12), H₂ alone (Comparative Example 13), and H₂ added in excess of O₂ (Comparative Example 14). Even when H₂ and O₂ added were mixed in the optimum ratio, the treating performance of H-type zeolites, such as H mordenite (Comparative Example 15) and H-ZSM-5 zeolite (Comparative Example 16), other than γ-alumina, was low.

In Comparative Example 12, γ-alumina changes into AlF₃ according to the following reaction scheme, and deteriorates:

2Al₂O₃+3CF₄→3CO₂+4AlF₃

When γ-alumina deteriorates, its catalytic activity declines and its life shortens.

On the other hand, when H₂ is added in such an amount as to be not less than the amount of the fluorine atoms of CF₄, the fluorine atoms in CF₄ are converted to HF. Thus, no AlF₃ forms, and there is no decline in the catalytic activity of γ-alumina.

TABLE 4 Treating Concentrations Concentrations Amount agent layer Total gas in inlet gas in outlet gas of CF₄ Treating temp. inflow CF₄ O₂ H₂ CF₄ CO CO₂ treated agent (° C.) (sccm) (%) (%) (%) (ppm) (ppm) (ppm) (l/l) Ex. 6 γ-alumina 800 408 1.02 3.0 2.1 510   5 9690 52 Ex. 7 γ-alumina 900 408 1.01 3.0 2.1 510   5 9590 47 Comp. Ex. 11 γ-alumina 700 408 1.02 3.0 2.1 510 1000 8690 3.9 Comp. Ex. 12 γ-alumina 800 408 1.02 3.0 0 510  100 9590 4.5 Comp. Ex. 13 γ-alumina 800 408 1.03 0 4.2 500 4900 4900 4.9 Comp. Ex. 14 γ-alumina 800 408 1.02 3.0 4.2 500 4700 4900 5.8 Comp. Ex. 15 H mordenite 800 408 1.02 3.0 2.1 510 5100 4600 2.1 Comp. Ex. 16 H-ZSM-5 800 408 1.03 3.0 2.1 510 1000 8700 1.7 zeolite

Example 8

Test for treatment of CF₄ was conducted using Neobead GB-08 (a product of Mizusawa Kagaku, principal component: γ-alumina) as γ-alumina.

The same testing equipment as in Example 1 was used, and CF₄, H₂ and O₂ gases were passed under the conditions described below. The outlet gas was analyzed where necessary, to investigate the treating performance of CF₄. The experimental conditions are shown below.

A nitrogen gas containing CF₄, H₂ and O₂ of the concentrations shown below was continuously passed for 1 to 2 hours. Then, a nitrogen gas containing H₂ and O₂ of the concentrations shown below was introduced for about 1 hour to discharge F remaining in the treating agent as HF. Then, a nitrogen gas containing CF₄, H₂ and O₂ of the concentrations shown below was continuously passed again for 1 to 2 hours. This cycle was repeated.

<Experimental Conditions>

{circle around (1)} Inflow concentrations:

CF₄ 1% H₂ 3.0% O₂ 5.7%

The amount of H₂ introduced was equivalent (2%) to the amount of F in CF₄, and O₂ was in excess of H₂.

{circle around (2)} Total gas flow rate: 408 sccm

{circle around (3)} Amount packed: 49 ml (25 mmΦ×100 mmh)

{circle around (4)} SV: 500 hr⁻¹ (corresponding to 125 L/min for a 15L column)

{circle around (5)} Treating agent: Neobead GB-08 (a product of Mizusawa Kagaku, principal component: γ-alumina)

{circle around (6)} Heater control temperature: 900° C.

<Test Results>

The results are shown in Table 5. The following facts were demonstrated thereby:

{circle around (1)} CF₄ could be decomposed and removed at a rate of 100% during the period of 380 min.

{circle around (2)} Then, the removal rate gradually declined, but a removal rate of 80% was obtained after a lapse of 720 min.

{circle around (3)} CO, CO₂, HF, H₂, and SiF₄ were present as by-product gases. The amounts of CO and H₂ were in the several ppm range.

{circle around (4)} CO₂ was formed in an amount nearly equivalent to that of the carbon atoms of CF₄, so that CF₄ was completely decomposed.

TABLE 5 Results of CF₄ Treatment Test Treating Outlet Removal Time CF₄ CO CO₂ HF H₂ O₂ total-F total-Si rate of (min) (ppm) (ppm) (%) (ppm) (ppm) (%) (mg/m³) (mg/m³) CF₄ (%) 0˜5 <1 <2 1.03 <1 <2 4.3 <1 <1 100  5˜10 <1 <2 0.91 <1 <2 4.4 <1 <1 100 10˜15 <1 <2 1.01 <1 <2 4.3 <1 <1 100 15˜20 <1 <2 1.02 <1 <2 4.3 <1 <1 100 20˜25 <1 <2 0.99 <1 <2 4.3 <1 <1 100 25˜30 <1 <2 1.02 <1 <2 4.5 <1 <1 100 30˜35 <1 <2 1.04 <1 <2 4.3 <1 <1 100 35˜40 <1 <2 1.02 <1 <2 4.4 <1 <1 100 40˜45 <1 <2 1.00 <1 <2 4.3 <1 <1 100 45˜50 <1 <2 1.06 <1 <2 4.3 <1 <1 100 50˜55 <1 <2 1.11 <1 <2 4.3 14 15 100 55˜60 <1 <2 1.00 2 <2 4.4 16 9.8 100 60˜65 <1 <2 1.01 5400 <2 4.3 180 150 100 65˜70 <1 <2 1.01 5400 <2 4.3 1820 500 100 70˜80 <1 <2 0.99 5400 <2 4.3 890 290 100 80˜90 <1 <2 1.00 5400 <2 4.2 1100 320 100  90˜100 <1 <2 0.99 5400 <2 4.2 2140 490 100 100˜110 <1 <2 1.02 5400 37 4.2 1350 500 100 110˜120 <1 <2 1.00 5400 63 4.2 1320 470 100 120˜130 <1 <2 1.02 150 4.0 4.0 590 190 100 130˜140 <1 <2 1.07 140 4.9 4.2 560 190 100 140˜150 <1 <2 1.07 450 4.2 4.2 1560 550 100 150˜160 <1 <2 1.10 2100 4.1 4.2 8760 2240 100 160˜170 <1 <2 1.07 2600 4.0 4.2 11700 3180 100 170˜180 <1 <2 1.10 3500 3.6 4.3 13000 3860 100 180˜190 <1 5 1.07 3500 4.1 4.1 13700 3460 100 190˜200 <1 5 1.05 2600 4.0 4.4 13600 3330 100 200˜210 <1 2 1.09 2600 4.3 4.2 12300 3430 100 210˜220 <1 10 1.08 5400 5.5 4.2 12300 3250 100 220˜230 <1 5 1.02 2100 21 4.3 13000 3220 100 230˜240 <1 8 1.03 5400 3.4 4.3 13300 3570 100 250˜260 <1 <2 1.00 3500 6.4 3.9 2160 780 100 270˜280 <1 <2 0.99 10400 7.8 4.1 9920 2330 100 290˜300 <1 5 1.02 10400 6.5 4.0 12500 3270 100 310˜320 <1 5 1.06 3500 7.1 4.2 2690 1770 100 330˜340 <1 5 1.04 10400 6.4 4.1 2610 1700 100 350˜360 <1 5 1.04 10400 6.1 4.1 29800 7850 100 370˜380 <1 <2 1.07 5000 5.1 3.9 1120 610 100 410˜420 4.1 6 1.06 800 3.8 3.8 1570 960 99.96 470˜480 54 5 1.04 950 4.2 4.0 64400 30000 99.46 490˜500 160 <2 0.90 3500 <2 4.4 1730 1230 98.4 530˜540 390 5 0.99 3500 2.2 4.2 10900 3850 96.1 590˜600 590 5 0.94 5200 5.2 4.3 27000 9890 94.1 610˜620 1020 3 0.97 10400 2.1 3.9 14100 3810 89.8 650˜660 1570 3 0.92 10400 83 4.0 2080 1830 84.3 710˜720 2110 3 0.90 10400 93 4.0 2270 840 78.9

According to the present invention, a fluorine-containing compound in an exhaust gas can be decomposed at a high decomposition rate. Furthermore, treatment can be performed for a long time, without generation of a large amount of CO which is hazardous and difficult to treat. 

What is claimed is:
 1. A method for decomposing a fluorine-containing compound in an exhaust gas by contacting the exhaust gas containing the fluorine-containing compound with aluminum oxide at a high temperature enough to decompose the fluorine-containing compound, and in the presence of such a necessary or larger number of moles of a hydrogen gas (H₂) that the fluorine atoms in the fluorine-containing compound become HF, and in the presence of an oxygen gas whose amount is not less than the number of moles of the hydrogen gas.
 2. The method of claim 1, wherein the contacting step is performed in the presence of 1.5 or more times as many as such a necessary number of moles of a hydrogen gas (H₂) that the fluorine atoms in the fluorine-containing compound become HF.
 3. The method of claim 1, wherein the fluorine-containing compound contains a carbon atom, a sulfur atom, or a nitrogen atom, and the contacting step is performed in the presence of such a necessary or larger number of moles of an oxygen gas (O₂) that the carbon atom becomes CO₂, the sulfur atom becomes SO₂, or the nitrogen atom becomes NO₂.
 4. The method of claim 3, wherein the necessary or larger number of moles of the oxygen gas has 1.5 or more times as many as the necessary number of moles.
 5. The method of claim 1, wherein the aluminum oxide includes γ-alumina.
 6. The method of claim 5, wherein the aluminum oxide substantially consists only of γ-alumina.
 7. The method of claim 1, wherein the aluminum oxide is particulate.
 8. The method of claim 1, wherein the high temperature is in a temperature range of about 800° C. to about 900° C.
 9. The method of claim 1, wherein the fluorine-containing compound includes C₂F₆, C₃F₈, CF₄, CHF₃, SF₆ or NF₃.
 10. The method of claim 1, wherein the exhaust gas has been discharged from a semiconductor device manufacturing apparatus.
 11. The method of claim 1, wherein a catalytic poison to the aluminum oxide is removed from the exhaust gas, and then the contacting step is performed.
 12. The method of claim 1, wherein SiF₄ is removed from the exhaust gas, and then the contacting step is performed.
 13. The method of claim 1, further including a step of removing an acidic gas after the contacting step.
 14. The method of claim 13, wherein the acidic gas is removed using water. 